This invention relates to the field of orthopedic implants, more specifically, a polymer PEEK (polyether ether ketone), carbon fiber PEEK, PEAK or hybrid (polymer and metal) based spinal rods which are used for deformity corrections with growth preservation in early-onset scoliotic patients and for the fusion of two or more vertebrae in degenerative spinal disease or deformity. This invention also relates to a mechanism for the minimally invasive distraction of such a rod.
Surgical techniques for the treatment of early-onset scoliosis are aimed at deformity correction with preservation of growth. The most common approach includes the use of distraction-based growth rods. A typical growth rod fixation has two foundations, namely, proximal and distal, where limited fusion is performed. Pedicle screws, hooks, wires, and other retaining structures may be used at each foundation to anchor the rods with the bony structure. Each foundation has a rod spanning toward the other end, which are connected to each other near the thoracolumbar junction. The rods are connected using a domino, which helps in distraction for serial surgeries until a final fusion is performed.
In a single growth rod technique, the rods span only one side (the concave side) of the spinal deformity curve. In a dual growth rod technique, the rods span both (both the concave and convex sides) of the spinal deformity curve. For the dual growth rod technique, the region of the rod near the foundation can have a crosslink connecting the rods of both sides.
In a typical growing rod implant surgery, the rods are attached along one or both sides of the spine above and below the spinal deformity curve using the pedicle screws or other retaining structures. The rod is then extended to correct the spinal deformity curve until the surgeon feels enough compression in the rod to stop the adjustment. The spinal deformity curve can usually be corrected by fifty percent at the time of the initial surgery. During the first operation, the patient usually undergoes invasive surgery. Regular construct lengthening is typically scheduled approximately every six months to a year thereafter. The lengthening procedure usually includes exposing the domino connectors through a small midline incision, loosening either the cranial or the caudal domino-connector setscrews, and distracting across the two rods within the connector. This lengthening process is frequently continued for a period of five to ten years after implantation.
Although known spinal and growth rod structures and installation procedures have functioned satisfactorily, several potential limitations have been determined. First, current spinal rods are made out of metallic materials, such as stainless steel, cobalt-chromium, or titanium. As a result, these metal spinal and growth rods are very rigid. The high level of rigidity of these metal spinal and rods may restrict the micro-motion of intervertebral disc in the spine, which can cause spontaneous fusion at the intervertebral junction. Spontaneous fusion is an undesirable clinical complication because the aim of the growth rod surgery is to delay or limit spinal fusion so as to allow the spine to grow. Second, current spinal rods are known to experience a relatively high rate of breakage. The material used to form the rod is an important consideration because it is a construct bearing load for a longer duration without spinal fusion, and its durability is vital. Third, the current surgical technique exposes the patient to a chance of infection because of the midline exposure of the tissue during subsequent distraction surgeries. Even though the use of minimally invasive techniques may reduce the chance of infection, none of the current mechanical distraction systems are simple enough for distraction, and some have many sub-units. Therefore, it would be desirable to avoid all of these potential problems.